Inteins are internal protein sequences that catalyze a protein-splicing reaction, which precisely excise the intein sequence and join the flanking sequences with a peptide bond. Inteins are embedded within a variety of host proteins termed exteins. Over 350 inteins have been identified in various proteins from bacteria, archaea, and eukaryotes. In a 2006 review (Saleh et al 2006 The Chemical Record 6:183-193), the authors state at page 189: “Most naturally or artificially split inteins are fragmented between motifs B and F (the position where the EN domain is found) yielding C-terminal intein fragments (IC) that are ˜25-40 aa, well within the scope of solid phase protein synthesis (SPPS), and N-terminal intein fragments (IN)>100 aa. Not all fragmentation positions in this region yield functional pairs. The Ssp DnaB mini-intein can also be reassembled from three pieces, including an In of only 11 aa. Fragmentation positions within the two long Ssp DnaB intein β-strands (β5 and β10) that form the backbone of the horseshoe structure yielded inactive intein fragments.” [internal citations omitted]
Inteins split into an N-terminal portion (N-inteins) and a C-terminal portion (C-inteins), which can reassociate non-covalently to form a functional intein, occur in nature and have also been engineered from contiguous inteins. Sun et al 2004 J Biol Chem 279(34):35281-35286 [42] reported an unsuccessful attempt to produce a small (6-amino acid) C-terminal intein from Ssp DnaB.